For the source text click/tap here: Bechorot 48
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According to this Mishna it appears that in cases where it is not clear whether or not there is an obligation to pay the five sela redemption fee to the kohen, there is no obligation to pay.
Thus, a father whose wife gives birth to twin boys knows that he must pay the kohen, even if they are unsure which child was born first. If one of the twins dies or if the twins were a boy and a girl, then it is not clear that the living boy was the firstborn, and no redemption must be paid.
We explore whether the law adjudicate monetary rights according to the actual configuration of ownership, or must it also recognize alternative legal arrangements that a litigant has the power to bring about?